Wood has been the principal source of fibers for use in making paper since the beginning of that industry. The rapid increase in demand for paper and the resultant rapid depletion of the supply of wood together with the slow growth of trees has compelled the paper-making industry to look for other sources of fibers that could be used in paper manufacturing.
One possible new source which has been investigated for the past several decades is bagasse, which is what remains after a sweet watery liquid has been expressed from sugar cane, as by passing the cane between rollers. The expressed liquid contains a disaccharide which, after being purified and concentrated, is refined to commercial sugar. Bagasse has been used mainly as a fuel for heating in processes where the expressed liquid is concentrated and refined.
Bagasse consists of about 45% to 47% by weight of alpha cellulose, from about 25% to 30% by weight of hemicellulose in the form of beta- and gamma-cellulose and from about 23% to 25% by weight of lignin. In order to soften, defiber and open the fibers and make a pulp suitable for paper manufacturing, paper makers have subjected bagasse to severe treatment, e.g., a digestion at a high temperature in a solution containing alkali and/or sulfur compounds.
One illustration of the foregoing prior practice is disclosed in U.S. Pat. No. 3,738,908 issued to Eduardo J. Villavicencio. The process there disclosed includes prehydrolysis of bagasse at a temperature of between about 171.degree. C. to 188.degree. C. with resultant decomposition of the xylose, formation of furfural, and loss of xylose. Such severe treatment removes large amounts of lignin and most of the hemicellulose, and these substances are discarded as black liquor, representing a loss of valuable materials. The only product obtained from processing bagasse in this manner is pulp which can be used in making paper.
Where the digestion step in prior processes leaves an excessive hemicellulose content remaining in the fibers, paper made from the resultant pulp has the appearance of parchment and is brittle. On the other hand, if not enough of the gamma- and beta-cellulose remains with the fibers, a sheet of paper made from the pulp does not possess desired strength.